The WAAS Ionospheric Spatial Threat Model

Eric S. Altshuler, Robert M. Fries and Lawrence Sparks

Abstract:	The Wide Area Augmentation System (WAAS) is a safety-critical, software-intensive system, augmenting the satellite-based Global Positioning System (GPS) in providing users with airborne positions of adequate accuracy, availability, and integrity during different phases of flight. Under the "Free Flight" concept of the National Airspace System (NAS) adopted by the Federal Aviation Administration (FAA) at the turning of the century, the GPS/WAAS infrastructure is assuming a critical role in ensuring the safe and efficient flight operating capability of the NAS. In the WAAS system, the ionosphere is modeled as a flat plane in the East and North direction (the radial direction is ignored). Grid Iono Vertical Errors (GIVEs) are calculated at iono grid points (IGPs) which make up a virtual grid 350 km above the surface of the earth. These GIVEs are to be used to estimate the overbound of a user error due to the ionosphere. The GIVEs must account for the deviations from this planar model that the user might experience if the IGP containing that GIVE is to be used in his calculation of his Vertical Protection Error (VPE). During nominal conditions, the ionosphere is described very well by the planar model, and a constant decorrelation term is sufficient to describe deviations from planarity. During ionospheric storm conditions, however, the planar model of the ionosphere breaks down, and does not adequately describe the ionosphere. In these situations, the user is protected by a storm detector. The WAAS system has as part of its algorithms, a chi-square detector which measures how much the ionosphere deviates from the planar model. This chi-square detector will trip (i.e., alert the user) if the ionosphere becomes poorly modeled by a plane. There is the possibility of an ionospheric storm that is not sampled or is sparsely sampled by the system, and thus, does not trip the chi-squared detector. The WAAS ionospheric threat model seeks to characterize deviations from planarity in worst case situations in which a chi-square detector has not yet reached a 99.9% threshold. We define this situation as “moderately disturbed ionosphere”. The deviations are calculated by using a variety of data deprivation schemes in which a selection of iono pierce points is withheld from the planar fits of the ionosphere, and are used as virtual users. The deviation of these virtual users from the planar fits are then measured, and these deviations from planarity are characterized by two geometric metrics. The metrics are the radius of the planar fit (furthest IPP from the IGP) , and the ratio of the distance from the centroid of the IPPs to the IGP divided by the radius. The two metrics appear to sufficiently describe the planar fit in that the radius is a measure of the absolute scale of the planar fit and the relative centroid metric is a measure of how skewed the IPP distribution is. Eight storm days were processed and the results are presented. Furthermore how these results apply to the WAAS algorithms is discussed.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	2463 - 2467
Cite this article:	Altshuler, Eric S., Fries, Robert M., Sparks, Lawrence, "The WAAS Ionospheric Spatial Threat Model," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 2463-2467.
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